Alumina producers are constantly striving to make the process of extracting alumina from bauxite more economical; producers want to be able to make as much alumina as possible at the lowest operating cost. One aspect of an alumina extraction process that directly impacts process economics is the dynamics of the grinding stage.
The Bayer process is the most common process used to produce alumina (Al2O3) from bauxite ore. In a typical Bayer plant grind circuit, the bauxite ore(s) are combined with recycled caustic liquor, to generate slurry containing 25 to 55% solids.
The comminution process used depends on the bauxite type and physical characteristics (e.g. mineral composition, particle size, hardness and toughness). In general, the bauxite slurry is wet ground in a mill or a combination of mills including a rod mill, ball mill or a hammer mill. Rod mills may be used first to break large and tougher sized particles down to between 5 to 2 mm in diameter. Various screen types may be located at the discharge of the rod mill or a separate down stream Trommel screen, may be used for specific sizing control. The oversized particles are then recycled back to the inlet of the rod mill, while the under size material continues toward the digestion area.
Again depending on the nature of the bauxite(s) used in the plant, the slurry may be ground further to meet specific particle size targets. This is generally done to ensure effective down stream de-silication (if needed) but ultimately for effective alumina extraction in the digestion section of the plant. Thus, following the rod mills, a series of screens or cyclones may be used to separate the slurry grind for further recycling and grinding or to remove the finest particles so these may be transferred directly to the slurry relay tank. Comminution of the remaining slurry is then effected in a ball mill to give bauxite particle sizes below 0.5 to 0.1 mm. Ideally, the ratio of bauxite to spent liquor, which is added at the grinding stage, is driven solely by the alumina content in the bauxite and the desired liquor productivity. If higher than acceptable viscosity of the ground slurry occurs, then the capacity of the grinding circuit can be limited in order to affect adequate grinding of the bauxite.
Several operating factors affect the throughput of bauxite in the circuit. These include the type and size of mill used, the specific media charge and the downstream sizing control used. Poor grinding efficiency may be caused by excess bauxite flow for the size of the mill, very high slurry viscosity, larger than desired initial bauxite sizes and or insufficient media charge. This can result in a higher amount of recycle in the mill and an increase in energy consumption. In some cases, two mills might be used where the initial design called for only one mill.
Furthermore, in open milling circuits where sizing control is limited, poor grinding efficiencies will lead to challenges in maintaining adequate suspension of the oversized particles in down stream storage tanks. This can lead to higher mixing demands in the slurry relay and desilication sections of the circuit. The remedy in such circuits is often to cut the throughput of the bauxite in the circuit and this can lower plant alumina production.
Over grinding of the bauxite can also occur from a number of factors, namely, lower than design flow rates, low bauxite solid charges, high media charges, or as a result of excessive recycling within the mill circuit, for example, due to excessive scaling in either the mill discharge screens, or in down stream sizing control operations. Over grinding wastes energy and increases the particle surface area and the bulk viscosity of the bauxite slurry. This can directly affect the down stream pumpability of the slurry. Furthermore, inherent for some bauxites, high slurry viscosity increases the “stickiness” of the slurry, which can negatively affect scaling rates on screens, and in vessels and agitators in the pre-desilication section.
Improvements in the efficiency of grinding in an alumina extraction process are addressed in this disclosure.